1. Field of the Invention
The present invention relates to a developing apparatus for developing an electrostatic latent image formed on an image bearing member, for example, and more particularly, it relates to a developing apparatus used with an image forming apparatus such as a copying machine, a printer, a facsimile and the like.
2. Related Background Art
FIG. 11 is a schematic sectional view of a conventional color image forming apparatus.
Such a conventional image forming apparatus includes a digital color image reader portion at its upper part, and a digital color image printer portion at its lower part.
In the reader portion, an original 30 is rested on an original platen glass 31, and a reflected light image from the original 30 obtained by exposure-scanning the original by means of an exposing lamp 32 is focused on a full-color sensor 34 through a lens 33, thereby obtaining a color decomposition image signal. The color decomposition image signal is sent, through an amplifying circuit (not shown), to a video processing unit (not shown), where the signal is processed, and then, the processed signal is sent to the printer portion.
In the printer portion, a photosensitive drum 1 as an image bearing member is supported for rotation in a direction shown by the arrow R1, and, around the photosensitive drum 1, there are disposed a pre-exposure lamp 11, a corona electrifier 2, an exposing optical system 3, a potential sensor 12, four developing devices 4 (4y, 4c, 4m, 4k), toner (on drum) detecting means 13, a transferring device 5 and a cleaning device 6.
The image signal from the reader portion is inputted to the laser beam exposing optical system 3, where the signal is converted into a light signal by means of a laser output portion (not shown), and the light signal or laser beam is reflected by a polygon mirror 3a and then is passed through a lens 3b and a mirror 3c to scan (raster scanning) a latent image on the photosensitive drum 1, thereby obtaining a light image E.
In the printer portion, during image formation, first of all, the photosensitive drum 1 is rotated in the direction shown by the arrow R1, and electricity is removed from the drum by the pre-exposure lamp 11, and then, the photosensitive drum is uniformly electrified by the electrifier 2. Then, for each decomposed color, the light image E is lighted to form an electrostatic latent image corresponding to the decomposed color on the surface of the photosensitive drum 1.
Then, for each decomposed color, the corresponding developing device 4 is operated to develop the latent image on the photosensitive drum 1, thereby forming a toner image (mainly including resin). The developing devices 4y, 4m, 4c, 4k are selectively approached to the photosensitive drum 1 by means of eccentric cams 24y, 24m, 24c, 24k to develop the respective latent images.
A recording material is conveyed from a recording material cassette 7 to the transferring device 5 by means of a conveying system, and then, the recording material is supplied, by the transferring device 5, to a transferring station opposed to the photosensitive drum 1. The toner image formed on the photosensitive drum 1 is transferred onto the recording material supplied to the transferring station. In this example, the transferring device 5 includes a transfer drum 5a, a transferring electrifier 5b, an absorbing electrifier 5c and an opposed absorbing roller 5g, an inner electrifier 5d and an outer electrifier 5e, and a recording material bearing sheet 5f is provided around a circumferential opening of the transfer drum 5a in a cylindrical form. The recording material bearing sheet 5f is formed from a dielectric sheet made of polycarbonate film.
The recording material conveyed to the transferring device 5 is absorbed on the recording material bearing sheet 5f by means of the absorbing electrifier 5c and the absorbing roller 5g so that the recording material is passed through the transferring station as the transfer drum 5a is rotated, and, in the transferring station, the toner image on the photosensitive drum 1 is transferred onto the recording material by the transferring electrifier 5b. 
During predetermined revolutions of the transfer drum, on the recording material absorbed on the recording material bearing sheet 5f and conveyed in this way, predetermined different color toner images are transferred from the photosensitive drum 1 in a superimposed fashion, thereby forming a full-color toner image.
In case of a four-color mode, when the transferring of four color toner images is completed, the recording material is separated from the transfer drum 5a by a separation pawl 8a, a separation push-up roller 8b and a separation electrifier 5h, and the separated recording material is discharged onto a tray 1 through a thermal fixing device 9.
On the other hand, after the transferring, residual toner remaining on the photosensitive drum 1 is removed by the cleaning device 6 for preparation for next image formation.
When images are formed on both surfaces of the recording material, after the recording material leaves the fixing device 9, a convey path switching guide 19 is driven, so that the recording material is temporarily introduced into a turn-over path 21a through a sheet discharge vertical path 20 and then is temporarily stopped there. Then, by rotating a reverse rotation roller 21b reversely, the recording material is advanced in a direction opposite to the introduced direction with a trailing end thereof directing forwardly, with the result that a front surface and a rear surface of the recording material is reversed and the recording material is stored on an intermediate tray 22. Thereafter, the recording material is conveyed to the transfer drum 5a again, where an image is formed on the other surface of the recording material in the same image forming process as mentioned above. Then, the recording material is discharged onto the discharge tray 10 through the fixing device 9.
The recording material bearing sheet 5f of the transfer drum 5a from which the recording material was separated can be contaminated by adhesion of paper powder scattered from the photosensitive drum 4, developing devices 4 and cleaning device 6 and adhesion of toner upon occurrence of sheet jam and adhesion of oil during the image formation. In such a case, the contamination is cleaned by a fur brush and a back-up brush, and an oil removing roller 16 and a back-up brush, which are opposed to each other with the interposition of the recording material bearing sheet 5f, for preparing for next image forming process. Such cleaning is effected in pre-rotation and post-rotation and is always effected after occurrence of the sheet jam.
Further, in this example, a transfer drum eccentric cam 25 is operated to drive a cam follower 5i formed integrally with the transfer drum 5a, with the result that a gap between the recording material bearing sheet 5f and the photosensitive drum 1 can be set to a predetermined gap at a predetermined timing. For example, during a stand-by condition or when a power supply is turned ON, the distance between the transfer drum and the photosensitive drum is increased, so that the rotation of the transfer drum can be effected independently from the rotation of the photosensitive drum.
Further, the developing device 4 (4y, 4c, 4m or 4k) performs the following developing operation in the image forming operation. When the electrostatic latent image reaches a developing station opposed to a developing sleeve 41 of the developing device 4 by the rotation of the photosensitive drum 1, developing bias obtained by overlapping AC and DC is applied to the developing sleeve 41 from a developing bias power supply (not shown), and the developing sleeve 41 is rotated in a direction shown by the arrow B by a developing sleeve driving device (not shown) and, pressurization is effected by a developing pressurizing cam 24 (24y, 24c, 24m, 24k). In this condition, the latent image is developed.
Further, when the gradation of the toner image formed on the photosensitive drum (toner image eventually formed on the recording material) is controlled (i.e., a half-tone image is formed), a density detecting patch latent image is formed on the photosensitive drum 1, and the patch latent image is developed with toner by applying the developing bias to the developing sleeve 41 to obtain a patch image density of which is in turn read by a patch sensor 13. If the read image density does not reach a reference value, an image signal level is adjusted. By such adjustment, an exposed portion potential on the photosensitive drum exposed by the exposing optical system for the image signal is adjusted, and, the density value of the developed patch image at this point is fed-back again to a CPU 300 as an initial reference value. By effecting such operation for each color, the gradation of each color toner image formed on the photosensitive drum can be optimized, thereby obtaining a good half-tone image.
Further, when an amount of toner replenished into each developing device 4 in order to keep the density of the toner contained in each developing device 4 substantially constant, a patch image having predetermined density is formed on the photosensitive drum 1 (for example, between areas where normal images were formed during the image formation), and, by controlling the amount of the toner replenished into the developing device 4 in order to always keep an output value of the sensor 13 constant, i.e., in order to keep the density of the toner contained in the developing device 4 substantially constant, a weight ratio (T/C ratio) between the toner and carrier is optimized to obtain the image having the optimum density.
However, in the case where the gradation of the toner image formed on the photosensitive drum is controlled and in the case where the amount of the toner replenished into the developing device 4 in order to keep the density of the toner contained in the developing device 4 substantially constant, if developing electrical fields (biases to be applied to the developing sleeve) for developing both patch latent images are selected to be the same, the load due to the developing electrical field acts on developer (toner), thereby shortening the service life of the developer.